Creating and using strong named assemblies in ASP.NET 5

The support for signing assemblies was first introduced to ASP.NET in the beta-6 release. In the upcoming RC1 release the way signing works has changed a little bit. In addition, from now on, assemblies that are part of ASP.NET 5 will be signed as well. Some people are questioning the decision of signing ASP.NET assemblies with strong name but there are good reasons to do it. First, some companies have a policy that all the assemblies they build must be signed. Without signing ASP.NET 5 could not be used in such environments since a signed assembly cannot depend on a non-signed assembly. Another reason is servicing – while it was never a goal to install ASP.NET 5 in GAC having signed assemblies leaves the option to use GAC to service ASP.NET 5 (e.g. in case of a critical security vulnerability) open.
There is a discussion about signing ASP.NET 5 assemblies where you can comment on the decision or ask questions.

In ASP.NET 5 there are three ways an assembly can be signed:

  • an assembly can be signed with a private and public key pair – this is just regular signing everyone knows and “loves”; assemblies signed like this can be installed in GAC etc.
  • an assembly can be delay signed – in this case the assembly is signed only with a public key but is really unusable unless you enable skipping strong name verification for this assembly using the sn tool (sn -Vr). The goal is to enable using assemblies signed with a private key to which developers don’t have access in test/dev environment.
  • an assembly can be OSS signed – this is a new way of signing that is very similar to delay signing in that the assembly is signed only with a public key. The difference is that it is no longer required to skip strong name verification for this assembly. The assembly just appears to be strong named while it really is not. (Internally OSS signing is oftentimes referred to as “fake signing” – in the Roslyn github repo there is even a tool called FakeSign). There are some scenarios that OSS signed assemblies cannot be used in – for instance an OSS signed assemblies cannot be installed in GAC. You can find more details on OSS signing and its limitations here.

If you want to sign an assembly in ASP.NET 5 you need to set appropriate compilation options in the project.json file. There are three settings that control signing:

  • keyFile
  • delaySign
  • useOssSigning

Depending on what combination of the signing options you use and the platform you are compiling on your assemblies will be signed in one of the ways enumerated above.
If you have just the keyFile in your compilation options (e.g. "compilationOptions": { "keyFile": "myKey.snk" }) the assembly will be signed with a private and public key pair (i.e. good (?), old signing) if you compile your project using Desktop Clr. CoreClr and Mono don’t support signing with a key pair so in these cases the public key will be extracted from the .snk file and will be used to OSS sign the assembly.
If you have both the keyFile and useOssSigning (e.g. "compilationOptions": { "keyFile": "myKey.snk", useOssSigning: true }) the public key will be extracted from the .snk file and the assembly will be OSS signed with the public key regardless of the runtime you use for compilation.
If you have just useOssSigning set to true, the assembly will be OSS signed with a hardcoded public key that is part of the ASP.NET 5 runtime.
If you set both useOssSigning and delaySign to true, the useOssSigning wins and the assembly will be OSS signed.
The delaySign option is ignored if you are compiling with CoreClr or Mono or you the keyFile option is missing.

Side note: When we enabled signing in beta-6 I noticed that there was a common misconception that signing depends on the platform the code was compiled for and not the platform the code was compiled on. You could tell this because people used the keyFile and (now removed) the strongName options together to sign assemblies produced for desktop Clr using the key pair (.snk) and OSS sign assemblies for CoreClr (since signing with a key pair is not supported on CoreClr). This resulted in errors since these options were mutually exclusive. The thing is that assemblies are signed when they are compiled and to compile an assembly you just use one runtime. Once an assembly is compiled and signed it will work with any (desktop Clr, CoreClr or Mono) runtime. In other words – if you sign an assembly with the public/private key pair (which you can do only if you compile the assembly using Desktop Clr) it will run just fine on CoreClr or Mono (they basically don’t verify strong names). From this point of view using the keyFile and strongName options together did not make much sense since it was just like saying “I want this assembly to be signed with a key pair and also with a public key only). This confusion was also one of the reasons why the strongName option was replaced with the useOssSigning option – the new option can be used together with the keyFile option and is more powerful.

Being able to OSS sign assemblies with an .snk opens up a couple of very interesting scenarios. For instance, imagine an open source project where the assemblies are signed using a private and public key pair during build. The .snk file is part of the project and is checked in to the source control. This basically means that the project was meant to be compiled using Desktop Clr. Now, if you wanted to compile this project with Mono or CoreClr things will most likely break – signing with a key pair did not work on Mono or CoreClr (before ASP.NET 5 RC1) so the assemblies will not be signed at all. This in turn may lead to further failures – e.g. if you use the InternalsVisibleToAttributes they will have to be modified as well to remove the public key. So, before you even started working on a project you had to spend a lot of time to introduce changes that will have to be thrown away when you commit your actual changes. OSS signing with .snk files fixes this because now without any changes to the project the assembly will be automatically signed with the public key and all code that depend on this assembly will just work.
The second scenario is similar to the first one except that this time the .snk file is not part of the project. So, you can clone and build the project (let’s call it A) but the assembly will not be signed. This is fine if assemblies that depend on A don’t have to be signed. But if they do you would have to sign A. The problem is that you don’t have the .snk so you will have to create your own .snk. As a result all assemblies that use A now need to be changed and recompiled because the identity of A changed. This might be quite a task but is doable… if you have code for all these assemblies. If you don’t then you’re dead in the water – you won’t be able to do that. Again, OSS signing can fix this scenario. Just dump the public key from the original assembly to an .snk (with sn -e [assembly] [targetfile]), build A and OSS sign it with the public key you just dumped. Everything else will just work (less than known limitations of OSS signing like installing OSS signed assemblies to GAC etc.) because the assembly will have the same identity as the original one.

Some miscellaneous notes:

  • assemblies signed with a private and public key pair can depend on OSS signed assemblies
  • to distinguish an OSS signed assembly from a delay signed assembly run sn –Vf [assembly]. sn -Vf will return the following error for OSS signed assemblies: Failed to verify assembly -- Strong name verification failed.
  • if you use xUnit as your testing framework you won’t be able to run tests on OSS signed assemblies with default settings. You will run into a limitation of OSS signing where the assembly cannot be loaded into AppDomain where shadow copying is turned on. To fix this run the xUnit runner with the –noshadow switch
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